The objective of the proposed research is to obtain increased knowledge about opsonic activity of human serum and determine how opsonins related to host defense against bacterial, fungal and viral disease. Opsonic activity of whole serum, specific antibacterial antibodies, complement components and components of the alternative pathway of complement activation will be studied. The methods of study will include quantitation of opsonic activity by the Maaloe technique, by radioalbumin technique and radiolabelled bacteria technique. Comparisons will be made between serum with and without complement factors and properdin and C3 proactivator. Patient populations for study include newborn infants, low-birth-weight infants, patients receiving dialysis and patients with increased incidence of severe infections. Factors in serum found to be essential for opsonic activity will be separated and prepared. It is hoped that controlled trials of serum or serum factors will be started for patients with greatly increased susceptibility to infection during this grant period.